The proposed research is to understand the molecular mechanisms through which a soluble protein, twisted gastrulation (Tsg), regulates the early vertebrate embryogenesis. A main focus will be on the strategies that Tsg uses to modify the signals from the bone morphogenetic proteins (BMPs). BMPs are versatile growth factors that control multiple processes during early vertebrate development, such as the dorsal-ventral patterning and the morphogenesis and the organogenesis of various organs. The activities of BMPs are regulated by many secreted factors, including the BMP antagonist chordin and the chordin inactivating enzymes tolloid/Xolloid/BMP1. Recently, a new soluble component, Tsg, has been identified, which seems to modulate the BMP signals in a unique way. Tsg can block the BMP function in both zebrafish and Xenopus; it, however, also induces some defects in frogs that partially mimic the phenotypes of the BMP overexpression. The mechanisms for the actions of Tsg in vivo are not well understood. Preliminary studies performed in this laboratory demonstrate that Tsg is required both in the dorsal and in the ventral regions for normal frog embryogenesis; Tsg may cooperate with chordin dorsally to control the dorsoanterior development. Tsg is detected at the cell periphery in vivo, and overexpression of Tsg may affect the cell movement. The proposed research will test the hypotheses that Tsg, in complex with chordin and BMPs, may be involved in the transportation of the BMP ligands and thus modulate the BMP signals; and Tsg may regulate the vertebrate development through its influence on the cell migratory behaviors. In aim 1, the in vivo interaction of Tsg, chordin, tolloid-related proteases and BMPs will be further studied at the molecular level by the loss-of-function approach. In aim 2, the distribution of the BMPs in the ectoderm of early frog embryos in the presence or the absence of Tsg and/or chordin will be analyzed. In aim 3, the influence of Tsg on cell movement will be examined. In aim 4, the direct interactions of Tsg with different BMPs and other unidentified factors will be studied. The results from these experiments will provide important clues on the mechanisms of the Tsg function, and will contribute greatly to our understanding on regulation of the BMP signals by a network of the extracellular proteins.